Functional role of ectomycorrhizae on drought resistance and recovery of beech and spruce

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In the previous funding term, we found structure and functions of ectomycorrhizal (ECM) communities of to be gradually changed during three summer droughts imposed in a throughfall exclusion experiment at Kranzberg Forest (TEE). Drought effects on ECM communities of Picea abies (L.) Karst. were stronger than on ECM of Fagus sylvatica (L.) but only after three repeated drought years. Increasing drought frequency reduced vital fine roots and ECM community diversity, but not hydrolytic enzyme activities. Tree mixture positively influenced ECM diversity and fine root vitality even under extreme water limitation. A relative increase in long-distance type ECM and no increase of EA with saprotrophic potential under drought support current concepts that question C- limitation in trees upon drought. The actual synthesis of results on ECM diversity and functions suggests correlations to vary depending on a given context of drought specific tree species reactions varying with monospecific/mixed interaction zone and soil depth. This high context dependency for survival of ECM requires further clarification. Therefore, a major objective in this follow up proposal is to better link above- and belowground processes upon drought resistance and recovery. Therefore this joint proposal of plant ecophysiologist and mycorrhizal ecologist aims to connect the rather contrasting above-ground drought responses of beech and spruce with below-ground differences in C supply under drought and to sinks upon re-watering. A phytotrone experiment is performed in the context of differently adapted ECM communities from different soil origins (moist vs. dry sites along a Bavarian precipitation gradient), and in parallel the unique conditions of the Kranzberg Forest TEE represents a relevant scenario on adult forest trees. 13C labeling in these experiments will allow addressing quantitatively C-allocation to sinks. For this purpose we will assess reactivity of ECM under drought and upon re-watering particularly the effect of mixture on attenuation of drought impact on ECM communities. This will be connected with tree sensitivity of and resource fluxes (C, H2O, N) among intra- and interspecifically grown beech and spruce under drought and subsequent recovery in differently adapted soils. We assess plant structural, physiological, biochemical parameters and use stable isotope labelling (13C, 15N, D2O). ECM community structure will be analyzed combined with functional parameters. A focus will be on parameters related to carbon relations of plant roots with fungi (invertase/amylase activity, composition of non-structural hydrocarbons (NSC)). The expected outcome of the proposal is clarification of 1) soil impact on drought response of trees, 2) effects of mixture upon drought reactions of tree species and their ECM communities, and 3) differences in drought recovery of beech and spruce upon re-watering.